Sex ratios (cont.)
Sex ratios and the demographic transition
The demographic transition is a concept describing how societies make a transition from high mortality and high fertility rates, to low mortality and low fertility rates. The decline in fertility rates often lags behind the decline in mortality rates, causing a period of high population growth in the intermediate stages of the transition. This causes an ageing of the population, where the proportions of older people increase while proportions of children and young people decrease. This is considered in greater detail in Session 1 (Demography on the World Stage) 
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The demographic transition has consequences for sex ratios:
- The sex ratio at birth increases slightly due to improvements in nutrition and medical care which means that more pregnancies reach full-term (see Section 4, Page 2 )
- The sex ratio for the total population, which tends to fall in a range of 90-110, tends to decrease over the course of the demographic transition. This is because the proportion of people at the oldest ages (where sex ratios are lower) increases. This is exacerbated by a shift in the predominant causes of death which affect men more than women (see below).
- The decline in the sex ratio with age changes during the demographic transition. As countries shift from high mortality to low mortality, there is a concurrent shift in health profile. Infectious and nutritional diseases are superseded by chronic diseases caused in a large part by lifestyle, and deaths from causes related to pregnancy and labour fall dramatically. The combination of men’s greater risk from lifestyle diseases, and reduced mortality for women due to safer pregnancy and childbirth, means that the decline in sex ratio is more rapid than in high mortality settings where the high environmental burden of disease affects everyone.
Sex ratios for major world regions, by 5-year age group, 2010.
Source: UN Population Division